The present invention relates to an image scanning device that is provided with a linear image sensor and a scanning system for capturing a two dimensional image of an object.
A conventional image scanning device of this type includes a linear image sensor, such as a CCD sensor (Charge Coupled Device sensor), which includes a predetermined number of photodiodes (representing pixels) arranged in a line and aligned in a predetermined direction. The image scanning device also includes an imaging lens for forming an image on the image sensor. The image sensor captures an image of one line (a scanning line) of the object and the scanning system operates to scan the object with the linear image sensor such that the linear image sensor captures a plurality of scanning lines until an image of the entire object is captured.
Generally, the scanning system operates such that the image sensor is driven in a direction perpendicular to the alignment direction of the image sensor.
In one conventional image scanning device, the imaging lens is combined with the image sensor as a scanning unit. The scanning system then operates such that either the scanning unit is driven to scan the object ("flatbed scanner") or the object is driven such that it is scanned by the scanning unit ("portable-style scanner"). In the first case, the object is placed, for example, on a transparent plate, and then the scanning unit is moved relative to the transparent plate. Generally, the transparent plate is located closer to the scanning unit, and the range of movement of the scanning unit generally must be as large as the object, such that the image scanning device is large and bulky. In the second case, since the object is moved past the scanning unit, a structure for moving the object must be provided. A well-known example of such a scanner is generally provided with a sheet feeder, and the object is generally a sheet of paper. Also in this case, the object moves past at a relatively close position to the scanning unit.
There is also known another type of scanning system with which an image of an object apart from the scanning system can be captured. In such a scanning system ("camera-style scanner"), an imaging lens is provided that forms an image of the entire object and then the image sensor is moved along a line representing the position of the image formed by the imaging lens. In such a system, the imaging lens is required to have an effective diameter corresponding to a diagonal line of the object. Such an image scanning device may be bulky and the imaging lens may be expensive to manufacture. Further, the system must have enough internal space to allow the movement of the image sensor and the moving mechanism therefor.
Further, when using a linear image sensor in a camera-style image scanning device, an image of the object may be subject to shading near the ends of the linear image sensor, and this shading should be corrected in order to obtain a high quality image. Also, since the object distance may vary within a wide range, the shading correction may vary depending on the object distance.
Still further, in a color camera-style image scanning device that uses color filters to generate a color image, an image of the object for a particular color may be subject to variations in brightness near the ends of the linear image sensor, due to variations in wavelength transmittance of the filters according to incident angle. Thus, color correction should also be performed in order to obtain a high quality image.